Powder lacquers based on epoxy-functional binding agents are known and are the subject of a large number of publications (see, for example, EP-B 0 509 393, page 2, lines 6 to 17). Polycarboxylic acids, including in particular the aliphatic dicarboxylic acids, the anhydrides and polyanhydrides thereof, or even carboxy-functional polyesters or polyacrylates, are described as examples of suitable crosslinkers for powder-lacquer binding agents of such a type. Of the large number of possible crosslinker components that have been proposed, however, dodecanedioic acid finds application almost exclusively in practice nowadays. Powder coatings cured with this dodecanedioic acid are distinguished at comparatively moderate stoving temperatures by very good optical properties, in particular by outstanding flow. Therefore such powder-lacquer systems have also been employed in the meantime as clear lacquer in the initial lacquering of automobiles.
The particular suitability of dodecanedioic acid as a powder-lacquer crosslinker is attributed, inter alia, to the very low solubility, in the solid state, of the crystalline acid in glycidyl-functional copolymers, particularly glycidyl methacrylates (GMA). By reason of its sharp melting-point, which lies within the range of the crosslinking-temperature, and by reason of its special melting behaviour, dodecanedioic acid enables, in combination with typical GMA resins, the production of powder coatings that, in comparison with those which have been cured with azelaic acid or sebacic acid, for example, exhibit a superior hardness, superior gloss and also better mechanical and chemical resistances (see, for example, Technical Information “Dodecanedioic Acid (DDDA): Curing Agent Applications in Acrylic Powder-Coatings”; DuPont Nylon Intermediates and Specialties; October 1997).
However, the exclusive concentration on dodecanedioic acid as crosslinker component for epoxy-functional powder-lacquer binding agents is not unproblematic for the technical application. Given a base of raw material that has been restricted in such a manner, bottlenecks in supply, or even simply fluctuations in the quality of the raw material, can very easily lead to an impairment of the provisioning of the lacquering plants with adequate amounts of powder lacquer. Furthermore, the high cost of dodecanedioic acid has until now precluded greater market success of acid-cured GMA powder lacquers.
The object of the present invention was therefore to make available novel carboxy-functional crosslinker components for epoxy-functional powder-lacquer binding agents, said crosslinker components being based on readily available and inexpensive raw materials and curing in combination with the customary GMA resins so as to form coatings that correspond, at least in their quality, to those which have been crosslinked with dodecanedioic acid.